For a window glass provided in front of a driver of a vehicle such as an windshield for an automobile, a laminated glass is usually employed, which is constituted by two glass sheets opposed to each other and an interlayer made of a resin sandwiched between them. Further, a technique of display device for a vehicle is known, which is a head-up display (hereinafter referred also to as HUD) displaying a display information in the front view field of a driver, such as an windshield.
As a conventional HUD device, there is one as shown in FIG. 16, in which a light beam 52 of display information projected from a projector 51 of a HUD device 50, is reflected by a point A on a surface in a car-interior side of an windshield and forms a display image (virtual image) 54 at a forward position outside the car, and another light beam 53 of display information is incident at a point B on the 1st face in a car-interior side of the windshield, refracted and reaches to a point C on 1st face of the windshield. The light beam of display information is partially reflected by the point C, incident at the point A of the 1th face, refracted and forms a display image (virtual image) 55, that is visible to a driver 26. However, in the HUD device 50, there is a demerit that the display image 54 formed by the reflection at a surface in the car-interior side of the windshield and a display image 55 formed by reflection at a surface in the car-exterior side of the windshield, are shift from each other to be visible as double images (so called ghost).
As means for preventing such double images, a method of making the position of display image in a distance to make the shift of the double images smaller than the resolution of eyes, a method of providing an optical rotator between a car-exterior side glass sheet and a car-interior side glass sheet of the laminated glass to make display light incident into the car-exterior side glass sheet at a Brewster Angle of P-polarization to prevent reflection, a method of making the angles of a car-interior side surface and a car-exterior side surface of the windshield different so that the windshield has a wedge-shaped thickness profile to make light paths of display images from the respective surfaces the same, etc., are proposed (for example, Patent Document 1).
On the other hand, in recent years, according to increase of demand for high-functional windshields for improving light-weight or amenity of e.g. automobiles, windshields having various functions have been devised. For an interlayer in a laminated glass for such a high-functional windshield such as a laminated glass having an increased ability to insulate sounds (hereinafter also referred to as laminated insulated glass), often a multi-layer interlayer formed by laminating resin layers having various functions is employed. Further, on a surface of an interlayer of a laminated glass, an emboss is usually provided for prevention of blocking phenomena, and for improvement of workability or improvement of deaeration property at a time of preliminary press-bonding.
In such a laminated glass employing a multi-layer interlayer, often optical distortion occurs, which causes a problem of distorting through-the-glass image. To cope with this problem, a method has been proposed, in which the shape of emboss on a surface of the interlayer is changed, and an emboss shape not generating optical distortion is formed on interfaces between the resin layers in the interlayer (for example, Patent Document 2).
Patent Document 1: JP-A-2-279437
Patent Document 2: JP-A-11-35348
The method of forming the windshield to have a wedge-shaped thickness profile as means for preventing double images of a HUD device, has such merits that light utilization efficiency is high as compared with other means for preventing double images, that the construction of the device is simple, or that visibility of a driver is not deteriorated since there forms no border line in the windshield.
However, when such a multi-layer interlayer is used for a windshield having a wedge-shaped thickness profile to constitute a HUD device, there has been a problem that due to optical distortion caused by the multi-layer interlayer in the display area, the display quality is deteriorated.
Particularly, in a case of employing the multi-layer interlayer as it is expanded in a substantially trapezoidal shape being the plane shape of the laminated glass, it is necessary to control the thickness of the multi-layer interlayer so as to have a wedge-shape at the same time of the expansion. At this time, there has been a problem that an optical distortion shows up in a portion where the interlayer is thin, which is caused by the interlayer. This problem is significant in a high-functional windshield typified by a laminated insulated glass, employing a multi-layer interlayer having a layer softer than commonly used polyvinyl butyral (hereinafter also referred to as PVB) being the material of the most commonly-used interlayer.
Further, in order to satisfy the demand for light-weight of vehicles, laminated glasses for vehicles tend to be thinner year after year. Accordingly, when the above-mentioned multi-layer interlayer is employed, restriction of the thickness of each layer constituting the interlayer becomes more strict. As a result, it has become extremely difficult to solve both of two problems at the same time, that are to employ a high-functional windshield and to prevent double images in a case of HUD device employing an windshield having a wedge-shaped thickness profile.
To cope with this problem, it is an object of the present invention to provide, in a HUD device employing a high-functional windshield having a multi-layer interlayer, an windshield for a vehicle which does not generate double images of display and is excellent in visibility, and a head-up display device employing such an windshield.